A token ring computer network includes a plurality of stations connected to a data transmission path, or trunk, in the form of a closed loop. A block diagram of a conventional token ring network is shown in FIG. 1. Trunk coupling units (TCU's) 10, 12, 14, 16, 18, etc. are connected in series on a ring 20. Stations 22, 24, 26, 28, 30 receive and transmit signals through TCU's 10, 12, 14, 16, 18, respectively. A token ring network can include up to 250 stations. The ring 20 can comprise shielded twisted pair, unshielded twisted pair, optical fiber, or any other suitable media (for example, microwave).
Information is transmitted on the token ring network in frames. Token ring networks operate such that only one station at a time transmits data. The frame containing the data includes source and destination addresses. After transmission of data has been completed by one station a token circulates on the ring 20 until another station seizes the token and begins transmitting data.
The token ring network uses a synchronous data transmission technique wherein the clock and data are combined in the transmitted signal. As used herein, the term "signal" refers to the combined data and clock utilized in token ring networks. Although the clock is recovered from the signal, the data and clock remain combined except within the station. A clock transition occurs at the center of each data bit position. A transition or a lack thereof at the beginning of each bit position indicates a one or a zero in the binary data. In the insert mode, the transmitted signal is received by a TCU from its upstream neighbor and is forwarded to the station. The station receives the signal and recovers the clock and data it contains. The station then combines the two to form a new signal and retransmits the signal to its TCU. Its TCU in turn forwards the signal to its downstream neighbor. In bypass mode, as contrasted with insert mode, the TCU forwards the signal it receives from its upstream neighbor directly to its downstream neighbor. This process is repeated until the signal reaches the active monitor. When the active monitor receives the signal, it sinks the recovered clock and stores the recovered data in a FIFO buffer. The active monitor then builds a new signal with the data and its own internal clock. The new signal then goes through the same repeating process as it did before the active monitor until it reaches the station which a initially transmitted the data. Along its path, the signal data content may also have been altered if the signal passed its destination, where it will have been marked as received, or if a station detected an error in the data content of the signal, where it will have been marked as erroneous. The operation and signalling in token ring networks is governed by IEEE 802.5/1989 standard for token ring systems.
One of the stations in the token ring network is designated as an active monitor. The selection of the active monitor station is established in an arbitration process when the system is turned on, when the active monitor station ceases performing the active monitor functions, or under certain error conditions. Thus, different stations can function as the active monitor at different times. All other stations on the network function as standby monitors. The active monitor sources and sinks the clock that circulates throughout the token ring network. The standby monitor stations have the capability of generating the clock. However, when a station is in the standby monitor mode, it recovers the clock and data from the signal issued by its upstream neighbor and transmits a signal composed of the recovered clock and data to its downstream neighbor.
A block diagram of a station in a conventional token ring network is shown in FIG. 2. Incoming signal passes through an equalizer 40 to a phase locked loop 42 and a demodulator 44. The phase locked loop 42 recovers the clock from the incoming signal and controls the demodulator 44. The recovered data is input to a FIFO buffer 46 if the station is the active monitor. The recovered data and clock are supplied to a MAC layer and SMT within the station after buffering for the active monitor. The output data from the station is supplied by the MAC to a modulator 48 which combines the data and clock to form a signal which it inputs to a transmit filter 50. The modulator 48 is controlled by the recovered clock from phase locked loop 42 when the station is a standby monitor. The modulator 48 is controlled by a clock 52 (active monitor clock) when the station is the active monitor.
A number of interoperability problems have arisen in token ring networks. One of the main interoperability problems involves the operation of the phase locked loop circuits used for clock recovery. The conventional token ring network as described above consists of a chain of up to 249 phase locked loops driven by a master oscillator/clock in the active monitor station. The standard modulation technique used to encode and transmit information on the physical media combines both data and clock in a composite signal of electrical or light pulses. The station receiving the signal must recover the clock and data from the signal. Most stations use a single clock recovery circuit for both reception and transmission functions. The design of the circuit is a compromise between fast tracking ability for the received signal and low jitter content for the transmitted signal. As stations recover and retransmit the clock to downstream neighbors, distortions can occur. The distortions are dependent on implementation details and can accumulate in a very complex manner. Since the performance of each of the 250 phase locked loop circuits in a token ring network is interdependent, performance requirements on these circuits is critical. The most severe problems are clock jitter and accumulated phase slope. If these problems become sufficiently severe, system errors and system failures can occur.
In a token ring network, trunk coupling units, stations, ring media and media between the TCU and stations can be supplied by different vendors. Since the IEEE 802.5/1989 standard for token ring networks does not define how these units are implemented, the analysis of a system supplied by mixed vendors is extremely difficult. The signal received by a particular station in the network depends on all upstream stations up to the active monitor.
One prior art attempt at reducing clock coupling in the token ring network utilizes dual phase locked loops in the station. This approach breaks the overall coupling chain and apparently provides performance improvements. However, each station is still affected by its upstream neighbor and still affects its downstream neighbor. The clock coupling between stations is not broken. Furthermore, this approach can be used only in new stations and does not improve the performance of existing stations. Such stations may insert or withdraw from the ring at will and their benefits on an existing ring are thereby not assured unless practically all stations on the ring include this feature.
In general, there is a need for a token ring network architecture wherein the clock coupling between stations is broken so that deviations from normal operation do not propagate through the network. The architecture should be entirely compatible with existing stations. The principal requirement is to transmit data from station to station using frames which are relayed by every station in a synchronous clocked scheme.
It is a general object of the present invention to provide improved token ring networks.
It is another object of the present invention to provide token ring networks wherein clock coupling between stations is broken.
It is further object of the present invention to provide an improved concentrator for coupling a plurality of stations to a token ring network.
It is a further object of the present invention to provide improved methods and apparatus for coupling a ring to a station in a token ring network.
It is yet another object of the present invention to provide methods and apparatus for coupling a ring to a station in a token ring network which are compatible with existing stations.
It is a further object of the present invention to provide improved methods and apparatus for coupling to an existing token ring network.